Infection of crops by fungi is a well-documented problem that can significantly affect crop yields. Various treatments of fungi by synthetic (chemical) products (including both curative and prophylactic treatments) have been developed, but they present problems as well. One problem, for example, is the effect of the so-called chemical load on the environment, animal and human health, and food safety. For these and other reasons, the popularity of “organic” or “bio-” fungicides, i.e. those derived from natural sources, has increased in recent years.
An additional problem encountered with the use of synthetic antifungal compounds, noticed by the early 1970's, is that resistance to such antifungal compounds sometimes develops. For example, the classes of antifungal compounds known as DMIs and strobilurins (discussed below) are biochemically active on a specific target site, and their broad use has led to fungal strains becoming less sensitive to other members of these classes of compounds. In response, various methods for countering such resistance when it occurs, or for reducing the likelihood of such resistance even developing, have been developed to enable continued use of synthetic fungicidal compounds when possible. These methods, also called “resistance management strategies”, include, among other steps, the simultaneous or sequential use of combinations of synthetic antifungal compounds that have different modes of action, limiting the number of uses of a particular antifungal compound during a giving growing season, and applying a given antifungal at a dosage that equals or exceed a particular minimum dosage. See, e.g. Keith J. Brent and Derek W. Hollomon, “Fungicide Resistance in Crop Pathogens: How Can it Be Managed?”, 2nd revised edition, 2007, Fungicide Resistance Action Committee (FRAC), Croplife International, Avenue Louise 143, 1050 Brussels, Belgium, available online at http://www.frac.info/frac/publication/anhang/FRAC_Mono1_2007_100 dpi.pdf.
In addition to combining fungicides in order to delay or reduce the emergence of resistant strains, fungicides are also often combined in mixtures in order to widen the spectrum and extend the duration of antifungal activity; and to exploit synergistic interaction between the active fungicidal compounds, whereby the overall activity can be increased. Synergy, which is a frequent phenomenon in fungicide mixtures, may occur between antifungal compounds of different natures and sources, between fungicides with different or identical modes of action, and between those prepared in different formulations.
While in principle combinations of synthetic antifungal compounds could be used to reduce the chemical load of any particular synthetic antifungal compound applied to crops, the fear of resistance at such lower loads has mitigated against the use of such lower loads, as reflected in the FRAC paper referenced above.
In the case of bananas in particular, infection by Mycosphaerella fijensis, a fungus of the class ascomycetes, commonly known as Black Sigatoka, is a well-known and widespread problem. Currently, chemical treatment of Black Sigatoka is effected using one or a mixture of synthetic fungicidal compounds, which can be categorized into five major groups (although some fungicidal compounds used do not fall into one of these groups): (a) demethylation inhibitors (DMIs), (b) Amines, (c) Quinone outside Inhibitors (QoIs), (d) Anilinopyrimidines (APs), and (e) Benzimidazoles. According to the web page of the FRAC Banana Working Group on the FRAC website (http://www.frac.info/frac/index.htm), (i) the DMI fungicides presently used in banana cropping are bitertanol, difenoconazole, epoxiconazole, fenbuconazole, flusilazole, hexaconazole, myclobutanil, propiconazole, tebuconazole, tetraconazole and triadimenol (all of these compounds are triazoles); (ii) the amine fungicides presently used in banana cropping are spiroxamine, fenpropimorph and tridemorph; (iii) the QoI fungicides presently used in banana cropping are azoxystrobin, pyraclostrobin and trifloxystrobin (all which are belong to a class of molecules sometimes called strobilurins); (iv) pyrimethanil is the only active ingredient from the group of anilinopyrimidines currently used in banana cropping; and (v) the benzimidazoles presently used in banana cropping are benomyl, carbendazim, thiabendazole, thiophanate and thiophanate-methyl (although the latter two area actually thiophanates rather than benzimidazoles, but they are classed by FRAC with the benzimidazoles). These compounds are applied to the leaves of banana plants—the part of the plant where Black Sigatoka infections occur—by spraying a composition containing one or more of the compounds listed above as the active ingredient(s). Some dithiocarbamates (e.g. mancozeb) and spiroxamines (spiroxamine) are also used to treat Black Sigatoka.
The FRAC Banana Working Group sets out the following guidelines for all fungicides used to treat bananas: (a) for a combination of active ingredients to be effective in a resistance management strategy, the rate of application of each active ingredient must be sufficient to provide satisfactory control when used alone at the same rate; (b) the recommended label rate of each mixture component must be respected; (c) protectant (multi-site) fungicides are considered to be a very valuable and necessary tool for the banana Sigatoka control programs and resistance management; and (d) site-specific fungicides must be applied in oil or oil-water emulsions. Guidelines (a) and (b), alone and in combination, mean that banana growers do not reduce the dosages of fungicides, even when from a short-term economic and an environmental standpoint it would be desirable to do so, e.g. to reduce expenditures on fungicides or to reduce chemical run-off into the ground.
Similarly, other crop diseases caused by fungi of the class ascomycetes may be treated using synthetic fungicidal compounds, but here too the development of resistance is a concern. Emulsions containing tea tree oil (TTO) for fungicidal application to plants, e.g. to plant leaves, are known. See, for example, US Patent Publication No. 2007/0237837, and the commercial product available under the name Timorex Gold from Biomor Israel Ltd., P.O. Box 81, Qatzrin 12900 Israel, http://www.biomor.com/timorex%20gold.htm. Timorex Gold has been successfully applied to combat Black Sigatoka (see e.g. Eduardo and Reuveni, “A New Potent Bio-Fungicide For the Control of Banana Black Sigatoka”, Phytopathology 2009, abstract from APS meeting 2009), although neither TTO in general nor Timorex Gold in particular appear in the FRAC Banana Working Group's list of fungicides used to treat Black Sigatoka. Similarly, Timorex Gold has been used alone to combat ascomycetes fungi in other crops.